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Stock-Based Compensation - Stock-Based Compensation Expense (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]      
Total stock-based compensation expense $ 9,089 $ 15,057 $ 11,730
Research and development [Member]      
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]      
Total stock-based compensation expense 5,031 6,463 6,444
General and administrative [Member]      
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]      
Total stock-based compensation expense $ 4,058 $ 8,594 $ 5,286